300CD rear power window help

[swizz]

I'm in the middle of trying to figure out what is going on with mr rear driver side power window. When I first got the car the window didn't roll down all the way, just about 80% down. Today I was looking through the repair manual, and noticed the vertical bar in the picture with the arrow on it was pivoted in the wrong direction. I popped the bar off, reversed direction, and was able to roll the window all the way down, and then rolled it all the way up with no problems. Then I went to roll it back down, and about 20% of the way down I can hear the electric motor bogging down as if its stressed. I'm afraid to to to roll it down any further, i don't want to burn out the motor, or even worse, break the glass. Anybody have any ideas? I'm pretty sure the bar I flipped is now in the correct position. Maybe over the years the mechanism as becomes warped? Any help is appreciated.

[Jorn]

It's at least 15 years ago when I repaired a similar problem I had with my 300CD. What I remember is that one of the bars was bend inwards and would hit another part in the mechanism so it wouldn't go down further. I was never able to get it perfect again.

[jgrahamdmd]

I have had my ‘78 300 CD since 1986 and have always had the same problems with the rear power windows. A few years ago, I had new window regulators put in and they worked properly for about two years, then began to hang up while trying to lower as they always had. Here’s two ways I can get my rear windows to go all the way down:
1) “bump” the window button up and down repeatedly until getting past the hang up
2) place downward pressure on the top front of the glass, then lower the window; this almost always works, but occasionally need to combine with bumping the window button
This just seems like an inherent design flaw with the W123 coupes.

Jim
‘78 300 CD (420K miles; black with tobacco brown interior)

[JHZR2]

It’s critical to lube the track that the window rides in. Everything that’s a point of friction should be lubed with appropriate products.

But it’s a weak/poor design. Supposedly newer reinforced regulators were made, both by Daimler and by backyard mechanics, but there’s no consistency that they’ve actually fixed the problem, with some reports showing that they bend too...

[Alex E]

The W123 coupe rear window guide plates are a weak design. They bend where the arm mounts to. Bend them straight again and weld reinforcements to strengthen the area. See attached pix of what I did, welded angle iron to the plates. They’ll never bend again. Windows now go up & down perfect.

[Alex E]

Here are a few more pix to better visualize the reinforcement

[JHZR2]

Alex E,

How hard was it to just remove those guide plates like that? I’d really like to get mine modified.

Thanks!

[Alex E]

It was a fairly easy job but time consuming in total. The hardest part is removing the interior side panels as there are hidden screws and like the front door panel pocket or center console wood, you can break a plastic ‘connector’ quite easy if done wrong. If you search W123 coupe rear window problems or something along that line ( I can’t remember ) you’ll find a good write up on the procedure. Once you get in there it’s easy to remove the guide plates, clean & lube tracks & hinges. I used 60% Moly grease (Honda Gold Wing spline lube) on the hinge points & gears. Ceramic caliper lube on the guide rod itself.
Rear windows been going up & down smooth as silk for a year now.

[Mike D]

The reinforcement suggested by Alex E. is a very good idea and I've done a similar repair on a couple of W123 coupes.

Don't overlook the cast aluminum motor brackets. They get "tweaked" and are quite often the cause of the guide failure.

I built a jig to bring the brackets back into shape. I made slow, small adjustments and an oven to reshape the brackets.

Used a 3/16" flat steel plate with drilled holes matching the mounting holes. Stripped the mount to bare metal, mounted the mount loosely to the plate, added washers/shims as required to raise low areas, used C-clamps on the high areas, gently tightened the mounting bolts and popped it into the oven at about 300F for a couple of hours. Let it cool, loosened the mounting bolts and clamps, added more shims, re-arranged the clamps and repeated the process until it was back into shape. Took a couple of days to get it reformed.

Probably wouldn't have taken as long if I had a dedicated oven. Sneaking the assembly in and out of the kitchen oven when the wife wasn't around added quite a bit of time.

[gonesurfing]

I had multiple issues with mine. Not only were the brackets bent, but my entire regulator assembly was bowed. this is on a '78.

I welded in an 1/8" gusset on each side of the bracket tab to keep it from bending, but had to replace both regulator assemblies anyway. The cast aluminum was bent and the teeth were jammed and binding on the gears. The replacement regulators had reinforced frames. I believe they were from an '82.

I lubed them and had them working well earlier this year. However, despite the copious amounts of waterproof silicone grease applied, they are getting sticky again. I'm back to using slight hand pressure to keep them from binding.

Oh, and never break that spring loaded plastic tab that rides in the front rail. They are NLA, and darned hard to find now. One replacement on flea-bay ran me $80. I tried rapid prototyping a non spring-loaded replacement, but it kept binding. Probably the Mercedes engineers tried that first too.

[JHZR2]

Hmmm... need to get this done for mine for sure.

I don’t weld. Willing to learn, or farm this out. Is it a newbie task?

[Alex E]

Originally Posted by JHZR2

Hmmm... need to get this done for mine for sure.

I don’t weld. Willing to learn, or farm this out. Is it a newbie task?

You can cut, shape, fit the reinforcing pieces - the time consuming part - and bring it to a welder.

[gonesurfing]

Yes, and be aware the bracket plating is possibly a cadmium plating. Based on the age and lack of rainbow luster, I think mine were. Don't breathe grinder dust or weld fume from it.

[t walgamuth]

Ahhh yes. I had trouble with mine too back when I had the CD. I finally decided to put them all the way shut by helping with my hand and didn't open them again.